Stack high water mark monitoring in FreeRTOS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the stack high water mark in FreeRTOS?

The stack high water mark is the minimum amount of free stack space left during the lifetime of a task. It helps identify how much stack a task actually uses.

beginner

Which FreeRTOS API function returns the stack high water mark for a task?

uxTaskGetStackHighWaterMark() returns the minimum amount of stack space that has remained for a task since it started.

intermediate

Why is monitoring the stack high water mark important in embedded systems?

It helps prevent stack overflow by showing if a task's stack size is too small, which can cause crashes or unpredictable behavior.

intermediate

How can you interpret a very low stack high water mark value?

A low value means the task used almost all its allocated stack, so you might need to increase the stack size to avoid overflow.

beginner

What is a practical way to check the stack high water mark during runtime?

Call uxTaskGetStackHighWaterMark() inside the task or from a monitoring task and log or display the value for analysis.

What does uxTaskGetStackHighWaterMark() return?

A Task priority level

B Maximum stack size allocated to task

C Minimum free stack space left since task start

D Current CPU usage of the task

uxTaskGetStackHighWaterMark() returns the smallest amount of free stack space that has been available to the task since it started.

Why should you monitor the stack high water mark in FreeRTOS?

A To detect potential stack overflow risks

B To measure task execution time

C To check memory leaks

D To adjust task priority

Monitoring the stack high water mark helps detect if a task's stack is too small and might overflow.

If a task's stack high water mark is very high, what does it mean?

A The task uses very little stack space

B The task is about to overflow its stack

C The task has crashed

D The task priority is too low

A high stack high water mark means a lot of stack space remained unused, so the task uses little stack.

When is the best time to check the stack high water mark?

A Only before starting the task

B Only during task creation

C After the system shuts down

D During runtime, periodically or on demand

Checking during runtime helps monitor stack usage dynamically and catch issues early.

What could happen if a task's stack overflows?

A Task runs faster

B System crashes or unpredictable behavior

C Task priority increases automatically

D Stack size reduces automatically

Stack overflow can corrupt memory and cause crashes or erratic system behavior.

Explain what the stack high water mark is and why it is useful in FreeRTOS.

Think about how much stack space a task actually uses versus what was allocated.

Describe how you would use uxTaskGetStackHighWaterMark() to improve system reliability.

Consider monitoring and reacting to stack usage data.

Practice

1.

What does the stack high water mark indicate in FreeRTOS?

easy

A. The total stack size allocated to a task

B. The minimum amount of free stack space a task has had

C. The maximum CPU usage of a task

D. The number of tasks currently running

Solution

1. Step 1: Understand stack high water mark meaning

   The stack high water mark shows the smallest amount of unused stack space a task has had since it started.

2. Step 2: Compare with other options

   Other options describe CPU usage, total stack size, or task count, which are unrelated to stack high water mark.

3. Final Answer:

   The minimum amount of free stack space a task has had -> Option B

4. Quick Check:

   Stack high water mark = minimum free stack space [OK]

Hint: Remember: 'high water mark' means lowest free stack left [OK]

Common Mistakes:

• Confusing stack size with stack usage
• Thinking it measures CPU or task count
• Assuming it shows maximum stack used

2.

Which is the correct way to get the stack high water mark of the current task in FreeRTOS?

?

easy

A. uxTaskGetStackHighWaterMark(NULL);

B. uxTaskGetStackHighWaterMark(currentTask);

C. getStackHighWaterMark(NULL);

D. uxGetStackHighWaterMark();

Solution

1. Step 1: Recall function signature

   The FreeRTOS function to get stack high water mark is uxTaskGetStackHighWaterMark, which takes a task handle or NULL for current task.

2. Step 2: Identify correct usage

   Passing NULL gets the current task's high water mark. Other options use wrong function names or parameters.

3. Final Answer:

   uxTaskGetStackHighWaterMark(NULL); -> Option A

4. Quick Check:

   Use NULL for current task in uxTaskGetStackHighWaterMark [OK]

Hint: Use NULL to refer to current task handle [OK]

Common Mistakes:

• Using wrong function names
• Passing invalid parameters instead of NULL
• Assuming no parameter needed

3.

Given the following code snippet, what will be the output if the task's minimum free stack space was 50 words?

UBaseType_t highWaterMark = uxTaskGetStackHighWaterMark(NULL);
printf("High water mark: %u\n", highWaterMark);

medium

A. High water mark: 50

B. High water mark: 0

C. High water mark: Stack overflow

D. Compilation error

Solution

1. Step 1: Understand uxTaskGetStackHighWaterMark output

   This function returns the minimum free stack space (in words) the task had, which is 50 in this case.

2. Step 2: Analyze printf output

   The printf prints the unsigned integer value returned, so output will be "High water mark: 50".

3. Final Answer:

   High water mark: 50 -> Option A

4. Quick Check:

   Function returns minimum free stack, printed as number [OK]

Hint: Function returns minimum free stack words, print as unsigned int [OK]

Common Mistakes:

• Assuming it returns zero on no overflow
• Thinking it returns error string
• Confusing with stack size or usage

4.

Identify the error in this code snippet intended to get the stack high water mark of a task named myTaskHandle:

UBaseType_t mark = uxTaskGetStackHighWaterMark(myTaskHandle);

Assume myTaskHandle is a valid task handle.

medium

A. Return type should be int, not UBaseType_t

B. Function name is incorrect, should be uxGetStackHighWaterMark

C. Parameter should be NULL to get any task's high water mark

D. No error, code is correct

Solution

1. Step 1: Verify function and parameter usage

   uxTaskGetStackHighWaterMark takes a task handle to get that task's high water mark. Using myTaskHandle is valid.

2. Step 2: Check return type and function name

   Return type UBaseType_t is correct. Function name is correct as per FreeRTOS API.

3. Final Answer:

   No error, code is correct -> Option D

4. Quick Check:

   Correct function and parameter usage means no error [OK]

Hint: Use task handle parameter for specific task, NULL for current task [OK]

Common Mistakes:

• Thinking NULL is required for all tasks
• Using wrong function names
• Confusing return types

5.

You want to monitor stack usage of multiple tasks and log a warning if any task's stack high water mark falls below 20 words. Which approach is best?

hard

A. Only check the current task's stack high water mark using NULL parameter

B. Increase all task stack sizes to 20 words to avoid overflow

C. Periodically call uxTaskGetStackHighWaterMark for each task handle and check if value < 20

D. Use vTaskDelete to remove tasks with low stack

Solution

1. Step 1: Understand monitoring multiple tasks

   To monitor multiple tasks, you must check each task's stack high water mark individually using their task handles.

2. Step 2: Implement threshold check

   Calling uxTaskGetStackHighWaterMark for each task and comparing to 20 words allows detecting low stack and logging warnings.

3. Final Answer:

   Periodically call uxTaskGetStackHighWaterMark for each task handle and check if value < 20 -> Option C

4. Quick Check:

   Check each task's high water mark against threshold [OK]

Hint: Check each task's high water mark, compare to threshold [OK]

Common Mistakes:

• Checking only current task misses others
• Assuming increasing stack size fixes monitoring
• Deleting tasks instead of warning